This invention relates to chart recorders and to their methods of operation.
A simple form of known chart recorder has a pen which is positioned in one direction in dependence upon the level of an input signal, as a sheet of paper is moved beneath the pen in an orthogonal direction, for example at a constant speed, so that the pen produces a trace on the paper. It is also known, if more than one trace is to be recorded at a time, to provide a corresponding number of pens, and in order to aid distinction of the traces from each other the pens may have different coloured inks. However, a problem with multi-colour pen chart recorders is that the pens cannot cross each other's paths unless they are arranged to draw on the sheet at different positions in the feed direction of the sheet, and thus the traces at a particular position in the feed direction are not contemporaneous unless delays are built into the driving of the pens in accordance with their relative positions in the paper feed direction.
With the advent of piezo printing technology, it is also known to provide a chart recorder with a single multi-colour dot-printing head (for example having six in-line pens for inks of six different colours) which is reciprocated in a scanning direction, while the recording paper is moved in an orthogonal sub-scanning direction, for example at a constant speed. With this example, six effectively continuous traces can be produced in the six different colours, by causing each pen to dot its particular coloured ink at a point during the scanning stroke of the head dependent upon the level of the respective input signal which is to be traced. In this example, if more than six channels are to be recorded at the same time, then one option is to use a head with more colours, but this adds to the expense of manufacture and maintenance. Another option is to use the same colour for more than one trace, and then perhaps to add labels to the chart to help distinguish between the traces in the same colour, but this is still likely to lead to confusion. A further option is to use ink-jet technology to form each dot as an array of pixels of different colours which are not readily distinguishable from each other by the naked eye. It is known to produce up to twenty-four different traces from four basic colours by such a dot pixel technique. However, with that many different traces it is not easy to distinguish some of the different-coloured traces from each other. Furthermore, a high resolution ink-jet head and high speed driving circuitry are required, resulting in high manufacturing cost. The present invention is concerned with these problems.